dd: work around buffer length restrictions with oflag=direct (O_DIRECT)

dd: work around buffer length restrictions with oflag=direct (O_DIRECT)

dd oflag=direct would fail to copy a file with size that is
not a multiple of 512 (destination file system specific)

* NEWS (Bug fixes): Mention it.
* src/dd.c (iwrite): Turn off O_DIRECT for any
smaller-than-obs-sized write.  Don't bother to restore it.
* tests/dd/direct: New test for the above.
* tests/Makefile.am (TESTS): Add dd/direct.
* doc/coreutils.texi (dd invocation): Mention oflag=direct
buffer size restriction.
Details in http://thread.gmane.org/gmane.comp.gnu.coreutils.bugs/17586
Reported by Eric Sandeen.